Why does LSD make you see things?

There is a scientifically correct answer, and one that can is a bit unprecise (scientifically speaking) but can be typed down in less than a week. And a basically wrong one, that at least gives a vague outline (last paragraph).

Simplified it works like this:

Everything you perceive (see, hear, feel, taste, smell, think, ...) you perceive through your nervous system, your brain. There are parts of your brain, that function like some sort of filtering-device. These parts (actual parts and functional networks) reduce the bandwidth of input and organize that input. If you get, say, 10 gigabit/s of input from your eyes those filters reduce that to less than 50 megabit/s of orgnized visual data. That is what you see when you see. What reaches your attentional focus, your 'consciousness'. The rest is filtered, cast away or goes straight to your unconcious, memory, etc.

LSD has an effect on these filters. It interferes with them functioning the way they normally do. As a result, the input that reaches that part of you, that you usually identify as 'myself', is massively increased.

What happens is that you either grok that input, take it as it is and find a way to comprehend the overload. Then all you get is there all the time, you just don't see it all the time. Because that would make you incapable of doing normal everyday things (smart pieces of wetware, them brains).

What happens, if you are not capable of serenely grokking that kind of mega-input - and hardly anyone is (though one can learn to)?

Then other parts of the brain, parts that recognize patterns and organize them, try to do what they always do (in this case as a sort of fallback, emergency function). They recall known patterns from long term memory and try to fit the mega-input into those patterns. For them to become manageable for your 'consciousness'. Under the overload it makes erroneous alocations and correlations:

Image you standing in front of a band-conveyor. The band moves with a speed of 1 foot per second and provides you with the wooden letters A and B and the cyphers 2 and 3. 10 cyphers or letters per minute. All your life your brain is in that situation, having to sort those letters into boxes labelled A and B, and the cyphers into boxes labelled 2 and 3. Contents of boxes A and B are supposed to be delivered to you 'consciousness', the focus of your attention. Contents of boxes 2 and 3 normally go to your unconscious.

Now a dose of LSD hits the throttle of that band-conveyor. Suddenly you get 1000 letters per minute at a speed of 600 feet per second. You (your brain), still try to function the way you usually do. In a way that has proven to keep you alive. A to box A, B to Box B etc.

But this is more your brain can handle (the way it usually works, at least).

Mistakes are made. 3 looks roughly similar to B. Now there are 3s in the B box and Bs in the 3 box.

Back to your real brain on acid. As in the story of sorting letters and cyphers, information not only floods your consciousness, but some 3 are mistaken for B (and vice versa).

Your brain still interprets the things it can not manage by fitting these things into something it believes it can. In extreme cases that's a hallucination then. E.g. a mega-input from your memory (maybe a childhood memory, because you smell something that reminds you) is mixed with a over-amplified feeling of eeriness (no wonder, you're on acid) and your brain arranges that input the best way it can and presents an emotionally distorted image of that nice old lady that used to live next door, when you were little. What you actually see seems to be something ork-like from a fantasy movie or a manga.

Or you see, for the first time in your life consciously, what would be a 3 in the letter-cypher-metaphor. It has been there all your life, it's dead normal averyday stuff to your unconscious. But to your 'consciousness' it's a mystic experience of knowing exactly what your buddy next to you thinks. While it's "only" an experience of hyper-empathy. (Mind you - it's still magic, even if you know how it's done!).

Sometimes the brain learns to see these bits of input it did not become 'conscious' of before and after the LSD has worn of, things still look different. (Learned for good then).

Don't panic. Now you can watch parts of your brain at work. Cool. Relax and enjoy. As long as it doesn't interfere with your everyday life, nice. Otherwise go see someone. Preferably a yogi, zen guy, bioenergetics practitioner, someone who understands what's going on and can handle it. Or you go see a psychiatrist, get a HPPD-diagnosis (hallucinogenic persisting perception disorder), therapy and/or tranquilizers. In extremly rare cases that actually makes sense. Mostly it's quite fun.

Ok. Here's the ultra-short answer:

LSD shuts down filters in your brain. You get info-overload. Your brain tries to make the best of it. Fits things into shapes it can handle. You see things. Voila.